I have reviewed the various 'Conditional Sum' questions in the forum and none quite match what I'm trying to do:

In the database, there is Date, Store#, Item#, and %Total Sales. In some cases, the same item# for the same date may be given more than one value for '% of Total Sales'. (For some reason this is a valid business scenario that happens rarely, but it happens.)

In that situation only, the requirement is to sum the two values together into one line. So if Item# 123 has a line with a value of .05%, and another line with a value of .08%, I need to sum those two values into one line for Item #123 that has a %Total of .13%. ONLY when an item has more than one percentage assigned, those percentages should be summed. Otherwise, if an item# has only one percentage value assigned, we just want to see that value.

I cannot figure out how to do this. Basically, I would like to implement logic that would work like this:

You can sum even if there is only one row - it will just sum that one row, resulting in the same value. See the example below. You can copy and paste it to an SSMS window and run it - you will see that it gives you exactly the output you are looking for: